Improved N2O capture performance of chromium terephthalate MIL-101 via substituent engineering

Abstract

The capture of N2O from CO2 by an adsorption-based mechanism remains a significant challenge due to their similar molecular sizes and physical properties. Although substituent engineering has been widely employed for the selectivity regulation and improvement of gas mixtures in metal-organic frameworks, its effect on N2O/CO2 separation has yet not been explored. Herein, four isoreticular MTN-type metal-organic frameworks (MOFs) with different substituents (-H, –NH2, -Br or –NO2) are reported for the capture of N2O from a N2O/CO2 mixture. As revealed by isothermal measurements, thermodynamic studies and ideal adsorbed solution theory computations, the N2O/CO2 separation potential can be effectively improved by an electron-donating group (-NH2), whereas electron-withdrawing groups (-NO2 or -Br) exhibit the opposite effect. Notably, MIL-101(Cr)–NH2 exhibits, by far, the highest ideal adsorbed solution theory equilibrium adsorption selectivity of 1.91 for 50/50 N2O/CO2. The N2O/N2 separation ability of the modified materials is also studied and functionalization with a -Br group yields an improved separation potential compared to the parent MIL-101(Cr) material. The strategy of using substituents to enhance the N2O/CO2 separation efficiency of MOFs could also be extended to other porous materials for chemical separation processes.